Aqueous solutions of polyacrylamide stabilized with thiocyanates



United States Patent 3,234,163 AQUEOUS SOLUTIONS OF POLYACRYLAMIDESTABILIZED WITH THIOCYANATES George F. Schurz, Oakley, and Keith R.McKennon,

Concord, Calif., assignors to The Dow Chemical Company, Midland, Mich.,a corporation of Delaware No Drawing. Filed Oct. 25, 1961, Ser. No.147,484

11 Claims. (Cl. 260-29.6)

This invention relates to acrylamide polymers. More particularly itrelates to compositions of such polymers having improved stability tothermal and/or oxidative breakdown comprising a small quantity of amember of the group consisting of alkali metal and alkaline earth metalthiocyanates and ammonium thiocyanate.

It has been observed that high molecular weight polymers and copolymersof acrylamide undergo degradation or molecular weight breakdown duringheating at elevated temperatures. It has also been observed that theactivity of water-soluble acrylamide polymers, when employed, forexample, as fiocculents in an aqueous medium, is often significantlyreduced within a few hours after dissolving the polymer. Similarly, ithas been observed that acrylamide polymers undergo degradation ormolecular weight breakdown during drying of an aqueous solution of suchpolymers, as well as during dry storage of these polymers.

While the exact mechanism by which such degradation occurs is not known,it is believed to be prompted by one or more agents such as oxygen andmetal ions capable of existing in two valence states such as, forexample, iron and copper and the like. Generally suspect are materialswhich are capable of forming or contributing to the formation of a redoxsystem. Such agents may be contained in the polymer or water asimpurities or deliberately incorporated therein for other purposes.Exemplary of the latter instance are the teachings in the United StatesPatent 2,960,486 wherein sulfites are addedto acrylamide polymers toreduce residual acrylamide monomer. The sulfites also preventdegradation of the polymer solution in the absence of air or oxygen.However, when exposed to an oxidizing agent, these sulfites have apronounced propensity to induce degradation of the dissolved acrylamidepolymer. Chain degradation or molecular weight breakdown in acrylamidepolymers is evidenced by decreases in the viscosity of aqueous solutionsof the polymer.

Accordingly, the primary object of this invention is to provide a noveland improved composition comprising an acrylamide polymer.

It is a further object of this invention to provide an acrylamidepolymer composition which is resistant to molecular weight breakdown dueto thermal and/or oxidative agencies in aqueous solutions as evidencedby losses in solution viscosity. It is still another object of theinvention to provide a solid acrylamide polymer composition resistant tomolecular weight breakdown as evidenced by losses in solution viscosity.Still another object of this invention is to provide a method forpreventing the thermal degradation of acrylamide polymers. Other objectswill become apparent from the following specification and claims.

In accordance with the present invention, water-soluble, solidacrylamide polymers, especially those having high molecular weights, arestabilized or at least improved in resistance to thermal and/ oroxidative degradation by intimately and uniformly incorporatingtherewith a small but effective quantity of an alkali metal thiocyanate,alkaline earth metal thiocyanate, or ammonium thiocyanate. To accomplishsatisfactory results, a water solution of the acrylamide polymer isprepared having "ice dissolved therein from about 0.1 to 7 percent byweight of an alkali metal thiocyanate, alkaline earth metal thiocyanateor ammonium thiocyanate based on the weight of polymer. Morespecifically, for aqueous solutions containing from about 1.5 to 2percent by weight of polyacrylamide, about 0.1 to 1 percent of thealkali metal thiocyanate, alkaline earth metal thiocyanate or ammoniumthiocyanate by weight of polymer is used, preferably about 0.25 percentby weight.

In aqueous solutions or gels containing up to about 50 percent by weightof polyacrylamide and in addition thereto, from about 1 to 2 percent byweight of polymer of a sulfite such as sodium sulfite, about 0.1 to 7percent by weight of the alkali metal thiocyanate, alkaline earth metalthiocyanate or ammonium thiocyanate is used, preferably about 0.25 to 1percent by weight of polymer.

The term acrylamide polymer as employed in the present specification andclaims refers to the homopolymers of acrylamide and water-solublecopolymers of acrylamide with other suitable monoethylenic monomerscopolymerizable therewith. These acrylamide polymers are water-soluble,vinyl-type polymers characterized by substantial linearity, i.e., havinglittle or no cross-linking between polymer chains. When the co-monomerpolymerized withthe acrylamide contains water-solubilizing groups, as,for example, in copolymers of acrylamide with acrylic acid, methacrylicacid, maleic acid, methacrylamide, vinylbenzylammonium compounds (suchas trimethyl-vinylbenzylammonium chloride), vinylbenzene sulfonic acid,N-vinyloxazolidinones, N-vinylpyrrolidinones, N-vinylmorpholinones andthe like, the copolymer contains up to about 50 mole percent of unitsderived from such co-monomers. However, with co-monomers containing nosuch hydrophilic groups, such as, for example, ethylene, butylene,styrene, alkyl esters of acrylic and methacrylic acid, vinylchloride,vinylidene chloride and the like, the copolymer contains no more thanabout 25 mole percent of the units derived from said co-monomers inamounts such that water-soluble copolymers result.

In carrying out the invention, the alkali metal thiocyanate, alkalineearth metal thiocyanate or ammonium thiocyanate is incorporated intosolutions of the acrylamide polymers in any convenient manner.

One procedure involves addition of the stabilizing component and theacrylamide polymer separately to a common aqueous medium. If the polymeris dissolved first,

it is desirable to utilize eflicient mixing devices such as mixingmills, pumps suitably designed or enclosed angersto accomplish acomplete and uniform mixing of the additives throughout the. polymersolution.

In another procedure, a sulficient quantity of the additive is firstuniformly blended with the acrylamide polymer to inhibit degradation inthe polymer when dissolved, and then utilizing the resulting compositionto prepare stable aqueous solutions.

Still another mode of carrying out the invention involves dissolving thestabilizing agent in an aqueous solution of the acryamide polymer andsubsequently recovering a solid homogeneous dispersion of thestabilizing agent in acrylamide polymer by evaporating the water ofsolution. Temperatures employed in any evaporation process utilized toaccomplish this result should be maintained below about 120 C.,preferably about C. in order to prevent an inordinate amount ofcross-linking of the acrylamide polymer. Above C., cross-linkingresulting in the formation of water-insoluble materials increasessubstantially.

In a preferred mode of operation, an aqueous solution containingsuitable amounts of an alkali metal thiocyanate, alkaline earth metalthiocyanate or ammonium thiocyanate is mixed with an aqueous solutionof; the acrylamide polymer in relative proportions so as to provide theultimately desired concentration; of the stabilizing component in theaqueous polymer solution. The result ing composition can be heated at atemperature of about 100 C. to evaporate the water of solution if therecovery of a solid product is desirable.

The aqueous acrylamide polymer solutions stabilized in acordance withthis invention exhibit ismaller decreases in viscosity in-the presenceof oxygen, ferric ions and like degradative agents, over Wide ranges inpH. Thelviscosity stability is directly correlated with a reducedpropensity of the acrylamide polymer to undergo molecular Weightbreakdown by thermal and/ or oxidative agencies.

The following examples are illustrative of the present invention {bill'-arenot to be construed as limiting its scope;

EXAMPLE 1 A quantity of 100 grams of an aqueous solution was prepared"by dissolving in water 1.5 grams of a water'- soluble polymercomposition consisting essentially .of a homopolymer of acrylamide}having about 4 percent of the amidegroups hydrolyzed to carboxylgroups,anda small amount of sodium bisulfite. The sodium bisulfite had beenintroduced intothe polymer composition in an amount of about 6.7 percentby'weight based on the polymer to reduce any free monomer present. Theabove-.

prepared composition is hereinafter designated Solution GiAIQ V To asecond acrylamide polymer solution, hereinafter above, was added 0.1gram of calcium thiocyanate;

' Both solutions thus prepared were placed in open bottles and heated atabout 70 C. to 80 C. for 2 hours in the presence of a continuous streamof oxygen.

Viscosities measured in centipoises were determined l for each polymersolution at 25 C. and at-pI-Iof 3.5 before and after heating by means ofa Brookfield vis-- cosimeter. The results of these measurements and the;percent loss of viscosity after heating are reported in the followingTable 1. The initial polymer solution viscosity before heating isdesignated as N and thatZ hours subsequent to heating is designated as NTable 1 Percent Loss of Polymer Solution N cps. N cps., Viscosity trz/NoXlOO) EXAMPLE 2 A homopolymer of acrylamide similar to that employed1n Example '1, which had been treated with Zpercent sodium sulfite, wasdissolvedinwvater; Enough of the polymer was dissolved to providelpercent by weight ofthe polymer in solution. The above-preparedcomposit1on is-hereinafter designated Solution A.

Toa second acrylamide polymer solution, hereinafter designated SolutionB, prepared identically to that above, was added 0.5 percentlby weightof polymer of sodium thiocyanate.

Both solutions thus prepared were placed in a open bottles atroomtemperature and air was bubbled through the solutions for a period of 72hours.

Viscosities measured in centipoise were determinedfo'reach polymersolution using procedures described inExample l.

The results of these measurements and-the percent loss of viscosityafter heatingaare reported in designated Solution B, preparedidentically to that,

Table ,2. The initial polymer solution viscosity before 1' heating isdesignated as N and that 72 hours subsequent to heating is designated asN A quantity of ml. of an aqueous. solution was preparedby dissolving inwater 3 percent by'weight of so- I dium chloride. and 0.05, percent byweight of a watersoluble polymer composition consisting essentially of ahomopolymer ofacrylarnide having about 30 percent of the amide groupshydrolyzed to:carbonylgroups. The. above-prepared Zcomposition. isihereinafte'r designated;

Solutionv A.

To a second acrylamide polymer solution,-hereinafter designated Solution.Bj prepared identically to :that' above, was addedlOO p.p.m.rofammonium thiocyanate. Both solutions thuss: prepared vwere placed inopen beakers and exposedto the atmosphere for a period of three days atnormal room temperatures.

Viscosities measured in centipoisesywere determined for each polymersolution at 25 C. and atzpH ranging from about. 6.9 to about 7.5 before"and after exposure by means of a Ubbelhode viscosimeter. Theyresultsofthese Y measurements and the percent loss of viscosity after exposurearereported in the following Table 3.: The initial polymer rsolutionviscosity before heating is designated as N and'that 3 dayssubsequent.toexposure isfdesignated as N;,. i

' Table 3' Percent v Loss of Polymer Solution No, cps. N3, cps.Viscosity od NOXIOO) EXAMPLE ,4'

A'suflicient amount of a homopolymer similar to that employedin Example1 is dissolved in :water-to form an aqueousgel containing 30percent byWeight of polymer,-

and 10 ml. of water issubsequently added... The, resulting gel and watermixture is thoroughly mixed and allowed to stand at room temperature forabout one hour, then dried for 90 minutes at C. in a standard laboratoryoven.

Theresultant dried,solid polymer is groundinto fine particles and theparticles dissolved in watertoform a 0.5

percent by weight solution." The above-prepared com: position ishereinafter designated Solution A33 To a second acrylamide*polymer'solution prepared j identically to; thatabove is added 10. ml;of a 10percent by weight aqueous solution of :sodium bisulfite.

described ,7 above. The above-prepared composition is hereinafterdesignated Solution .B., a

Tofla thirdacryl amide! polymer solution, prepared identically to.that-above .is; added 10 ml. of a 10 percent'by weight aqueous solutionof sodium. bisulfite containing about. 0.05 gram of calcium thiocyanate.The resulting.

solution is"imix'ed, ,rallowed to stand one hour, dried, ground. intofine. particles; and .the particles dissolved into water to form a 0.5percent by weight solution as described above. The.above-prepared:composition is here inafter designated Solution C'i Theresulting solution ismixed, allowed to;stand for one hour, dried, groundinto fine particles and-therparticles dissolved in water to form :a 0.5percent by weight solution as.

Viscosities measured in centipoise are determined for each polymersolution at 25 C. at a pH of 3.5 using an Ubbelhode viscosimeter.Solutions A and B described herein, sutfer a severe loss in viscosityafter drying, whereas Solution C, stabilized with calcium thiocyanate ischaracterized by significantly less viscosity loss after drying.

The acrylamide polymer compositions described in this invention tend tocross-link slightly during processing under the conditions described inthe preceding examples, such cross-linking resulting in a slightincrease in solution viscosity of the final product.

In a manner similar to that of the foregoing examples, water-solublecopolymers of acrylamide with one or more monomers such asmethacrylamide, vinylbenzylammonium compounds, vinylbenzene sulfonicacid, N-vinylpyrrolidinone, N-vinylmorpholidone, ethyl, butylene,styrene, ethyl acrylate, methyl methacrylate, vinyl chloride orvinylidene chloride are dissolved in water in the presence of a smallquantity of an alkali metal thiocyanate, alkaline earth metalthiocyanate or ammonium thiocyanate to achieve a comparable improvementin the resistance of acrylamide polymers to degradation.

What is claimed is:

1. A composition of a water-soluble acrylamide polymer and from about0.1 to about 7 weight percent of a member of the group consisting ofalkali metal and alkaline earth metal thiocyanates and ammoniumthiocyanate.

2. A composition of an aqueous solution of an acrylamide polymer havingdissolved therein from about 0.1 l

to about 7 percent of sodium thiocyanate based on the weight of thepolymer.

3. A composition as claimed in claim 2 wherein the acrylamide polymer ispolyacrylamide.

4. A composition of an aqueous solution of an acrylamide polymer havingdissolved therein from about 0.1 to about 7 percent of calciumthiocyanate based on the weight of the polymer.

5. A composition as claimed in claim 4 wherein the acrylamide polymer ispolyacrylamide.

6. A composition of an aqueous solution of an acrylamide polymer havingdissolved therein from about 0.1 to about 7 percent of ammoniumthiocyanate based on the weight of the polymer.

7. A composition as claimed in claim 6 wherein the acrylamide polymer ispolyacrylamide.

8. A composition of a solid acrylamide polymer in intimate admixturewith from about 0.1 to about 7 percent of a member of the groupconsisting of alkali metal and alkaline earth metal thiocyanates andammonium thiocyanate based on the weight of the polymer.

9. A composition as claimed in claim 8 wherein the solid acrylamidepolymer is polyacrylamide and the alkaline earth metal thiocyanate iscalcium thiocyanate.

10. In a method for improving the stability of an acrylamide polymer tothermally induced molecular weight breakdown in aqueous solutions, thestep of incorporating into the solution from about 0.1 to 7 percent byweight of a member of the group consisting of alkali metal and alkalineearth metal thiocyanates and ammonium thiocyanate.

11. In a method for improving the stability of solid acrylamide polymersto thermally induced molecular Weight breakdown, the steps of forming anaqueous solution of said polymer, incorporating therein, from about 0.1to 7 percent by weight of a member of the group consisting of alkalimetal and alkaline earth metal thiocyanates and ammonium thiocyanate,and recovering the solid admixture by evaporating the water of solution.

References Cited by the Examiner UNITED STATES PATENTS 2,578,653 12/1951Goppel et al 260-459 3,065,193 11/1962 Volk 260-459 MURRAY TILLMAN,Primary Examiner.

LEON J. BERCOVITZ, Examiner.

1. A COMPOSITION OF A WATER-SOLUBLE ACRYLAMIDE POLYMER AND FROM ABOUT0.1 TO ABOUT 7 WEIGHT PERCENT OF A MEMBER OF A GROUP CONSISTING OFALKALI METAL AND ALKALINE EARTH METAL THIOCYANATES AND AMMONIUMTHIOCYANATE.